Measuring Bandwidth using Open Source tools

Hello Everyone,
I came across one problem but so far i couldn't find good solution for my problem.
Here is the problem.....
I'm looking for a good open-source bandwidth measuring tool so that can measure throughput very well.
I tried MRTG but it only measure bandwidth usage but it is difficult to see the allocated bandwidth unless you add in the configuration file.

I really want to see if Service Provider truly allocated the agreed bandwidth for my site.

We have VPN, so i want to know what bandwidth is allocated without asking the service provider.